Conventional centrifugal pumps are composed substantially of a hydraulic part comprising a hollow body provided with suction and delivery ducts, containing one or more impellers intercalated by diffusers and keyed on the transmission shaft of an electric motor enclosed in a motor casing that isolates it from the pumping liquid and keeps it dry.
The electric motor must be further isolated from the liquids at the passage of the shaft for transmitting rotation to the impellers, ensuring the hydraulic seal between the liquid pumping region and the dry region of the motor.
For this purpose, mechanical seals are in widespread use which are to be mounted on the shaft to seal its passage to the impellers, of the single or double type and optionally of the cartridge type, fitted directly on the rotation transmission shaft, inside a sealing chamber that contains lubricant, in order to ensure correct lubrication among the parts that compose the seal, and is interposed between the motor and the pump body so as to surround the rotation shaft.
Submersed pumps and submersible pumps are particular types of pumps that belong to the centrifugal group.
In particular, the expression “submersed pumps” designates the type of pumps, usually having a vertical axis, designed to work underwater. Typical submersed pumps are well pumps, built to work at depth.
The expression “submersible pumps” instead designates pumps, also having a vertical axis, designed to be installed directly in the water on the bottom of a collection basin. Usually, these pumps work with the motor above the water level, but the electric motor and the coupling to the hydraulic part must be perfectly watertight, so that the centrifugal pump may work also submerged, and thus is submersible, if the water level rises in the collection basin. Submersible pumps are widely used in the lifting systems of sewers, in the draining of cellars, excavations and construction sites and in the transport of wastewater, since they are conceived to work with water full of suspended materials.
For these types of pumps, an optimum hydraulic seal is therefore fundamental to avoid compromising its functionality; however, the cited solutions are not devoid of drawbacks, such as, above all, the reliability of the system over time.
In fact, especially in the case of centrifugal pumps of the submersed type or of the submersible type, the seals are subject to wear over time and, due to the hydraulic pressure at which they work, let the liquid being pumped leak into the motor or into the lubricant chamber and then from there into the motor.
Moreover, these types of submersed pump include shredding pumps, typically for wastewater, i.e., provided with a shredder keyed on the rotation transmission shaft. The shredder is arranged in the lower region of the hydraulic part, where there is also an opening from which the liquid to be pumped is drawn. When the centrifugal pump is powered up, the shredder also is activated and, during the shredding of solid bodies, generates vibrations that affect the motor and particularly the bearings keyed on the rotation transmission shaft. These vibrations also put at risk the functionality of the mechanical seal, the parts of which tend to mutually lose alignment, allowing liquid to leak toward the motor. The longer the portion of shaft comprised between the shredder and the motor, the more the vibrations affect the latter.
Still, this type of centrifugal pump can be installed in lifting stations, in which sediments often form. The solid substances tend to compact over time, reducing the useful volume of the tanks and often clogging the pump. Their removal requires a targeted intervention that entails a prolonged and expensive stop of the pumping system. In order to facilitate the good operation of the centrifugal pump, operations for fluidizing the liquid to be pumped are therefore necessary and are often obtained by installing an agitator upstream of the pump body.